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Author Topic: Servo rewiring  (Read 2912 times)

FrankM

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Servo rewiring
« on: November 06, 2009, 09:23:05 am »

Hi guys,

Just looking for confirmation of a servo wiring issue.

I've picked up a couple of el cheapo servos that I intend to use to power the 6lb gun turrets on my 1/24 Fairmile D.  I've gone down the path of adding in the 2.2k resistors to get almost 180 degree travel - no problem.  However I intend to power them both from the same Tx stick using a Y lead from the Rx.  Clearly, wired as the come they will both turn the same way when instructed, i.e., clockwise or anticlockwise so one will train to port and one to starboard.

I want them to point either port or starboard together at the extent of their travel.

Am I right in assuming that if I simply switch the wires from the servo PCB to the servo motor on one of them, it's travel will be reversed thus giving me the desired outcome.

OR

Can I simply switch the +ve and -ve wires from the Rx to the servo.  Or does that stuff up the whole servo operation.

Of course I could just go ahead and experiment but I'd appreciate good input from forum members - might save me a servo..........

Thanks
Frank
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HS93 (RIP)

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Re: Servo rewiring
« Reply #1 on: November 06, 2009, 09:48:32 am »

the simple way is to get a "Y" lead fron the servo shop that reverses one side they are only cheap

http://www.servoshop.co.uk/index.php?pid=JP7721044&area=ACCS

peter
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stallspeed

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Re: Servo rewiring
« Reply #2 on: November 06, 2009, 10:14:04 am »

..........
Am I right in assuming that if I simply switch the wires from the servo PCB to the servo motor on one of them, it's travel will be reversed thus giving me the desired outcome.
No that won't work.You would have to switch the outer pot connections too.

Quote
OR

Can I simply switch the +ve and -ve wires from the Rx to the servo.  Or does that stuff up the whole servo operation.
That will actually reverse polarise the electronics and destroy the servo

Quote
Of course I could just go ahead and experiment but I'd appreciate good input from forum members - might save me a servo..........
Hitec servos all rotate clockwise with longer pulses on the orange/white (signal) wire
Most Futaba ones rotate counterclockwise so even with direct coupling you gan get servos with mirror image operation.
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FrankM

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Re: Servo rewiring
« Reply #3 on: November 09, 2009, 01:47:25 am »

Thanks guys.

Stallspeed,
So, looking at the servo, I have the black wire soldered to the case (-ve I presume).  The white and red wires go to the commutator lugs.  These I presume get the power input from the pot.  I'd like to understand why I can't simply switch the red and white wires at the motor.  Is there something different to an ordinary electric motor where just switching the input will cause it to run in reverse

HS93,
looked to the link you gave me.  I'm presuming there is some sort of electronic switching in this lead.  Does anyone know how it works.

At last a simple device that will make everyones life easier. Modern servos tend to all run in one direction and when placing two servos on one function this can lead to problems. This simple Y lead with a reverse module on one output - changes the direction of one servo - problem solved without having to turn a servo upside down etc.

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stallspeed

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Re: Servo rewiring
« Reply #4 on: November 09, 2009, 03:19:27 am »

Quote
Modern servos tend to all run in one direction
No,that is incorrect but it helps sales of the product if consumers are misinformed.
Like I said,most Futaba servos turn anticlockwise with long signal pulses on the white cable.
Hitec all turn clockwise.

Take your pick from these options.
  • If the servo matches a Hitec servo rotation then get one with Futaba rotation or vice versa
  • Buy the servo reversing lead (it has a heatshrunk circuit with an adjuster pot)

Don't swap the red and black cable because it will destroy the servo circuitry.
Swapping the red and white will just not work.
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dodgy geezer

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Re: Servo rewiring
« Reply #5 on: November 09, 2009, 09:27:19 am »

 I'd like to understand why I can't simply switch the red and white wires at the motor.  Is there something different to an ordinary electric motor where just switching the input will cause it to run in reverse...


I would guess that a servo is designed to take in a 'position signal', then run a motor in a direction that will match that signal to an internally generated 'position signal'.  If you reverse the servo motor connections, it will take in a position signal, then drive the servo in the opposite direction, and never stop....

The hitec/futaba answer sounds ideal, and servos come cheap nowadays - you can get some for under £2 at GiantCod. If you don't want that, your option is between the servo reversing lead, or adding a gear wheel to one turret drive....
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stallspeed

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Re: Servo rewiring
« Reply #6 on: November 09, 2009, 11:26:16 am »

There's nothing to stop you desoldering and reatatching the motor in opposite polarity and outer potentiometer connections.
You can go further by doubling the pot resistance (probably 5kΩ to 10kΩ) with two resistors equal to half the potentiometer value.
The servo will double its rotation.
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